The present invention relates to a color copying machine, a color printer, or a color facsimile apparatus that forms a color image including at least four (4) color components (e.g., yellow, magenta, cyan, and black). It finds particular application in conjunction with extending a color space defined using the four (4) color components, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
Acoustic ink printing ("AIP") uses acoustic radiation produced by an ejector to eject individual droplets on demand from a free ink surface. The droplets ejected from an AIP device are significantly smaller than those produced by devices used for other conventional printing methods (e.g., thermal ink jet printing). For example, a typical droplet produced by an AIP device is less than about ten (10) pico-liters (or often times even less than about two (2) pico-liters).
Relatively large spots make the conventional printing devices very capable of producing colors at the black end of the color gamut. However, the large spots also limit the production of highlight colors at the light end of the gamut (e.g., light grays) using only black printer ink.
Color in printed digital images results from the combination of a limited set of colors over a small area in "gray" values selected to integrate the desired color response. This is accomplished in many printing devices by reproducing so called "separations" of the image, where each separation provides varying gray values of a single primary color. When the separations are combined together, the result is a full color image.
The particular color of each separation depends on the "color space" being implemented. Two commonly used color spaces include red-green-blue ("RGB") and cyan-magenta-yellow ("CMY"). The RGB color space is additive (i.e., it uses the addition of select amounts of the primary colors to a black background, with an equal mixture of the three (3) primary colors producing white). In contrast, the CMY color space is subtractive (i.e., the cyan, magenta, and yellow inks remove the primary colors red, green, and blue, respectively, from light reflected off of a white background so that an equal mixture of the three (3) CMY inks produces black due to the absorption of all color).
In practice, color images are often printed in a cyan-magenta-yellow-black ("CMYK") color space. This color space is based upon the CMY color space, but attempts to improve the quality of "black" in the image and, at the same time, reduces the use of color inks. In theory, images can be printed using the CMY color space, with a mixture of the three (3) colors producing black. Printing with only cyan, magenta, and yellow inks, however, often results in a lighter output than black printer ink. Therefore, while the lighter black achieved from CMY is actually beneficial in the highlight region produced by conventional large-spot printers, the quality of dark blacks is diminished. The lighter output is due to impurities in the inks, the particular paper or other image recording media used, and the partial reflection of light instead of its complete absorption into the inks. Consequently, select use of black ink in place of the primary colors reduces expense and minimizes the total amount of ink used, which is often desirable in ink-jet and other printing applications where the ability of the recording substrate to absorb ink is limited.
Methods for converting from the CMY color space to the CMYK color space are commonly referred to as undercolor removal ("UCR") and gray-component replacement ("GCR"). UCR/GCR methods vary, but commonly involve examining the individual pixels of an image to determine the minimum of the three (3) cyan-magenta-yellow colors. One (1) or more of the three (3) colors are then adjusted according to the minimum amount of the three (3) colors (i.e., UCR). An equivalent amount of black ink is then added to account for the removal of the three (3) other colors (i.e., GCR). For example, if a given pixel of an image is represented in the CMY color space by C=0.5, M=0.4, and Y=0.25, then the black or K value would be based upon the lowest value (i.e., the Y value). In a 50% UCR/GCR method, K=50% of Y (i.e., K=50% of 0.25, or 0.125). The remaining CMY values would then each be reduced by 0.125 so that the resulting UCR/GCR pixel is represented by C=0.375, M=0.275, Y=0.125, and K=0.125. Of course, other UCR/GCR methods are known, but each seeks to determine the level of black for a given pixel, and to thereafter adjust the other colors accordingly to account for the addition of black ink.
UCR/GCR makes it possible to reduce the amount of consumption of cyan, magenta, and yellow inks and helps prevent over-saturation of the printing medium. Over-saturation is a concern with low-surface tension inks used in conventional printing devices. Furthermore, because the color of printer black ink is darker than the color of process black ("CMY black"), UCR/GCR makes is possible to produce a darker black ink than one produced by mixing cyan, magenta, and yellow inks. In other words, UCR/GCR extends the dark end of the color gamut.
Although UCR/GCR produces desirable results for conventional printing devices producing large spots and using low-surface tension inks, it limits dark colors from being produced using all four (4) printer inks in the CMYK color space. Furthermore, UCR/GCR limits the production of light-grays (i.e., highlight colors) from black printer ink alone. Small-spot printing devices, which typically use higher-surface tension inks, are capable of producing darker blacks using all four (4) of the printer inks in the CMYK color space and better highlight colors using black printer ink alone. Therefore, conventional UCR/GCR are not desirable printing methods for small-spot printing devices.
The present invention contemplates a new and improved method and apparatus for expanding a color space in acoustic ink printing which overcomes the above-referenced problems and others.